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[text: QQs: What makes fresh air fresh?]

Michael: It’s a warm spring day, so you open up the window and take a deep breath. Ahh! Fresh air.

But why does the air outside feel different than the air inside? And can you make indoor air fresher? Well, what makes outdoor air seem fresher is more about what’s not in it than what is. We mostly think about air pollution in terms of stuff that’s outside, like smoke or smog. But indoor air pollution is also a thing. Pet dander, mold, and bacteria all contribute to the dirtiness of air inside. Plus, carpets and furniture tend to slowly release organic compounds over time. And all that stuff makes the air smell stale. The more tightly sealed your home is, the more these pollutants can accumulate -- especially in energy-efficient homes, which tend to have poor ventilation.

I mean, sure, outdoor air can be polluted too. But there’s way more air outside your house than inside your house. So, usually, outdoor pollutants are less concentrated. That’s why, when you walk outside or open a window, the air feels nicer. There’s less garbage in it. If you want to freshen up the air without opening a window, you have options -- but they don’t work very well. Aerosol sprays and other scented products won’t do much to remove all that stuff from the air. Mostly, they just mask bad smells with nicer smells, some actually anesthetize your nose so it’s harder to smell bad odors.

That said, there are some air fresheners that do target organic molecules or bacteria in the air; the fanciest of these are air ionizers. Air ionizers produce electrons, which give air molecules a negative charge. Then, these negatively charged ions bump into and stick to pollutants in the air. That makes them heavier and more likely to be caught by an air filter, so you end up with less-polluted air -- in theory. There’s a significant downside to air ionizers.

Consumer Reports tested five models of air ionizers and found that they all produced pollution, in the form of ozone. Ozone is pretty hazardous to human health. It can cause chest pains and make asthma worse. Plus, the whole point of getting the air ionizer in the first place was to make the air less polluted, not more. So if you want a less hazardous way to freshen up your home, try growing some indoor plants, which can suck up air pollution both indoors and out. Or, you know, you could just open a window.

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